1. Field of the Invention
The present invention relates to improvement in or relating to a semiconductor photoelectric conversion device which has a conductive substrate or a first conductive layer formed on a suitable substrate, a non-single-crystal semiconductor laminate member formed on the conductive substrate or the first conductive layer, including at least one I-type non-single-crystal semiconductor layer and having formed therein at least one PI, NI, PIN, or NIP junction, and a second conductive layer formed on the non-single-crystal semiconductor laminate member.
2. Description of the Prior Art
Heretofore there have been proposed a variety of semiconductor photoelectric conversion devices of the type that has a conductive substrate or a first conductive layer fomred on a suitable substrate, a non-single-crystal semiconductor laminate member formed on the conductive substrate or the first conductive layer, including at least one I-type non-single-crystal semiconductor layer and having formed therein at least one PI, NI, PIN OR NIP junction, and a second conductive layer formed on the non-single-crystal semiconductor laminate member.
In the semiconductor photoelectric conversion device of such a structure, the I-type non-single-crystal semiconductor layer has the function of generating photo carriers corresponding to the incidence thereon of light. The I-type non-single-crystal semiconductor layer contains hydrogen or a halogen as a recombination center neutralizer, by which recombination centers are neutralized which would otherwise exist in large quantities since the I-type non-single-crystal semiconductor layer is formed of a non-single-crystal semiconductor. This prevents photo carriers created in the I-type non-single-crystal semiconductor layer from being lost by recombination.
The conventional semiconductor photoelectric conversion devices of this kind have a low photoelectric conversion efficiency of 8% or less.
As a result of various experiments, the present inventor has found that one of the reasons for such a low photoelectric conversion efficiency of the conventional photoelectric conversion devices is that when the I-type non-single-crystal semiconductor layer of the non-single-crystal semiconductor laminate member having the function of creating photo carriers is unavoidably formed to contain oxygen as an impurity, the oxygen content is as high as 10.sup.20 atoms/cm.sup.3 or more.
Further, the present inventor has found that another reason for such a low photoelectric conversion efficiency is that when the I-type non-single-crystal semiconductor layer is unavoidably formed to contain carbon as an impurity, the carbon content is as high as 10.sup.20 atoms/cm.sup.3 or more.
Besides, the present inventor has found that another reason for such a low photoelectric conversion efficiency is that when the I-type non-single-crystal semiconductor layer is formed to contain phosphorus, the phosphorus content as an impurity is as high as 5.times.10.sup.16 atoms/cm.sup.3 or more.
Moreover, the present inventor has found that when the oxygen content in the I-type non-single-crystal semiconductor layer is as high as 10.sup.20 atoms/cm.sup.3 or more, the semiconductor photoelectric conversion device provides a low photoelectric conversion efficiency of 8% or less for the following reason:
In the case where the I-type non-single-crystal semiconductor layer contains oxygen in as high a concentration as 10.sup.20 atoms/cm.sup.3 or more, a large number of clusters of oxygen are formed in the I-type non-single-crystal semiconductor layer and these clusters of oxygen serve as recombination centers of photo carriers.
Accordingly, when the oxygen content is as high as mentioned above, the I-type non-single-crystal semiconductor layer contains a number of recombination centers of photo carriers which are not neutralized by a recombination center neutralizer. Consequently, photo carriers which are generated by the incidence of light in the I-type non-single-crystal semiconductor layer are recombined at the recombination centers, resulting in a great loss of the photo carriers.
Further, the I-type non-single-crystal semiconductor layer, when containing oxygen, generates dangling bonds of oxygen, which serve as donor centers. In the case where the I-type non-single-crystal semiconductor layer contains oxygen in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more, it contains many dangling bonds of oxygen acting as donor centers. In this case, the midpoint level of the energy band in the I-type non-single-crystal semiconductor layer relatively shifts more towards the valence band than does the Fermi level. Accordingly, the efficiency of photo carriers generation based on the amount of light incident the I-type non-single-crystal semiconductor layer is very low. Further, the diffusion length of holes of the photo carriers in the I-type non-single-crystal semiconductor layer is short. This leads to low photoconductivity and very high dark conductivity of the I-type non-single-crystal semiconductor layer.
Moreover, when the I-type non-single-crystal semiconductor layer contains oxygen, the oxygen is combined with the material forming the layer. For instance, when the layer is formed of silicon, it has a combination with oxygen expressed as Si-O-Si. Accordingly, when the oxygen content is as high as 10.sup.20 atoms/cm.sup.3 or more, the layer contains a large amount the combination of the material forming the layer and oxygen.
On the other hand, the combination of the material forming the I-type non-single-crystal semiconductor layer and the oxygen contained therein is decomposed by the irradiation of light to create in the layer dangling bonds of the material forming it and dangling bonds of the oxygen.
Accordingly, in the case where the I-type non-single-crystal semiconductor layer contains oxygen in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more, the dangling bonds of the material forming the layer and the dangling bonds of oxygen, which are generated in the layer, greatly increase by the irradiation of light. In such as case, the dangling bonds of the material forming the layer act as recombination centers of the photo carriers, and the loss of photo carriers generated in the layer increases. When the number of dangling bonds of oxygen increases, the midpoint level of the energy band, which has shifted much further towards the valence band than the Fermi level, shifts further towards the valence band correspondingly, resulting in marked reduction of the photo carrier generating efficiency of the I-type non-single-crystal semiconductor layer. Also the diffusion length of holes in the I-type non-single-crystal semiconductor layer is further reduced, markedly lowering the photoconductivity and raising the dark conductivity of the layer.
When the photocarrier generating efficiency and the photoconductivity of the I-type non-single-crystal semiconductor layer have thus been lowered and the loss of the photo carriers in the layer and the dark conductivity of the layer have thus been increased, if the layer is heated, the dangling bonds of the material forming the layer and the dangling bonds of oxygen, generated in large quantities in the layer, are partly recombined with each other to re-form the combination of the material forming the layer and oxygen. As a result, the number of dangling bonds of the material forming the layer and the amount of oxygen decreases. In the I-type non-single-crystal semiconductor layer, however, the dangling bonds of the material forming the layer and the dangling bonds of oxygen still remain in large quantities. Consequently, the photo carrier generating efficiency and the photoconductivity of the I-type non-single-crystal semiconductor layer are very low and result a loss of photo carriers in the layer, and the dark conductivity of the layer is extremely high. In addition, the values of the photo carrier generating efficiency, the photoconductivity, the loss of photo carriers and the dark conductivity of the I-type non-single-crystal semiconductor layer differ significantly before and after irradiation by light and after heating.
The above is the reason found by the present inventor why the photoelectric conversion efficiency of the conventional semiconductor photoelectric conversion device is as low as 8% or less when the I-type non-single-crystal semiconductor layer contains oxygen in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more.
Further, the present inventor has found that when the I-type non-single-crystal semiconductor layer contains carbon in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more, the photoelectric conversion efficiency of the conventional semiconductor photoelectric conversion device is as low as 8% or less for the following reason:
When the I-type non-single-crystal semiconductor layer contains carbon in such a high concentration as 10.sup.20 atoms/cm.sup.3 as referred to previously, the layer forms therein a number of clusters of carbon. The clusters of carbon act as combination centers of photo carriers as is the case with the clusters of oxygen. Accordingly, when the I-type non-single-crystal semiconductor layer contains carbon in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more, a great loss of the carriers that are created in the layer due to incidence thereon of light results.
The above is the reason found by the present inventor why the photoelectric conversion efficiency of the conventional semiconductor photoelectric conversion device is 8% or less when the I-type non-single-crystal semiconductor layer contains carbon in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more.
Moreover, the present invention has found that when the I-type non-single-crystal semiconductor layer contains phosphorus in such a high concentration as 5.times.10.sup.16 atoms/cm.sup.3 or more, the photoelectric conversion efficiency of the conventional semiconductor photoelectric conversion device is as low as 8% or less for the following reason:
In the case where the phosphorus content in the I-type non-single-crystal semiconductor layer is as high as 5.times.10.sup.16 atoms/cm.sup.3 or more, the midpoint level of the energy band in the I-type layer shifts more towards the valence band than the Fermi level does, as in the case of the layer containing oxygen in such a high concentration as 10.sup.20 atoms/cm.sup.3 or more. Accordingly, the photocarrier generating efficiency of the I-type non-single-crystal semiconductor layer is very low. Further, the diffusion length of holes of the photo carriers generated in the layer is short, and hence the photoconductivity of the layer is low and its dark conductivity is very high.
The above is the reason found by the present inventor why the photoelectric conversion efficiency of the conventional semiconductor photoelectric conversion device is as low as 8% or less when the phosphorus content is higher than 5.times.10.sup.16 atoms/cm.sup.3.